1. Field of the Invention
The present invention relates to a beam scanner. More precisely, it relates to a high-precision laser beam straight line scanner having a holographic disk which scans a desired surface of a photoconductive drum.
In current beam scanners used to scan a photo-conductive drum in a laser printer with a laser beam, a conventional rotational polygon mirror, which is expensive and complex, has been replaced with a more easily manufactured, simpler and less expensive holographic disk. The beam scanner with a holographic disk realizes a "self-imaging" system without an auxiliary lens system, such as an f-0 lens, otherwise necessary to collect beams of light diffracted by the hologram of the holographic disk.
2. Description of the Related Art
The assignee of the present application proposed a holographic scanner as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 60-194419, in which a hologram is constructed by an interference of an objective wave and a reference wave (both being divergent spherical waves) which are illuminated from symmetrical point light sources with respect to a plane (line) normal to a reconstruction point of the hologram at which the hologram is to be reconstructed. Thus, when a reconstruction beam is incident upon the reconstruction point, a diffraction beam having a diffraction angle identical to the incident angle is produced, to thereby increase the allowable tolerance of deviation of the position of a rotating shaft of the holographic disk and an inclination of the plane of the holographic disk with respect to a horizontal plane.
FIG. 16 shows a hologram scanner disclosed in the above-mentioned Japanese publication, in which a hologram of a holographic disk 4 is constructed by an interference of a reference wave (divergent spherical wave) W.sub.1 and an objective wave (divergent spherical wave) W.sub.2, which are emitted onto a reconstruction point P at which the hologram is to be generated from points (light sources) A.sub.1 and A.sub.2 located in a substantially symmetrical arrangement with respect to a line (or plane) X normal to the plane of the holographic disk 4. When the holographic disk 4 having the thus-constructed hologram is rotated about a rotating axis 0, and a reconstructing beam is incident upon the reconstruction point P, scanning beams are diffracted by the hologram in predetermined directions, so that the scanning beams are traversed along a predetermined line (note: according to the invention, this line is not always necessarily a straight line) on an imaging surface (focal plane) T of a photoconductive drum 5. Parameters for designing such a straight line scanner (holographic disk) are normal distances f.sub.H1 and f.sub.H2 of the light sources A.sub.1 and A.sub.2 from the plane of the holographic disk 4 (f.sub.H1 =f.sub.H2), a wavelength .lambda..sub.1 of the constructing waves W.sub.1 and W.sub.2, a disk radial distance (incident distance) R, a wavelength .lambda..sub.2 of the reconstructing wave, and an incident angle .theta..sub.i of the reconstructing beam upon the holographic disk 4. The incident angle .theta..sub.i is given by the following equation: ##EQU1##
The design of the holographic disk can be based on the above parameters. Among those parameters, in particular, the most significant parameter is a ratio .lambda..sub.2 /.lambda..sub.1 of the wavelengths of the reconstructing beam and the constructing beams. Namely, a limitation is imposed on the kind of usable laser beams which must be fully coherent as a light source for constructing the hologram of the holographic disk. This inevitably leads to discrete characteristics of the hologram, as shown in FIG. 15. FIG. 15, shows the radial distance R (referred to hereinafter as an incident radius) and an imaging distance (focal length) l (FIG. 16) of an image from the reconstructing point P when a laser beam straight line holographic scanner is designed under the conditions that the wavelength .lambda..sub.2 of the reconstructing wave is fixed at 780 nm, which is a wavelength of a diode laser, and the wavelength .lambda..sub.1 of the constructing waves is one of 488 nm (Ar laser), 441.6 nm (He-Cd laser), and 325 nm (He-Cd laser), which are all commonly used to construct a holographic scanner. As can be seen from FIG. 15, to reduce the size of the holographic disk 4, i.e., to reduce the incident radius R while maintaining a constant local length l, the wavelength .lambda..sub.1 of the constructing wave must be decreased. This is difficult in that a limitation of the wavelength exists. For example, assuming that a desired focal length l is 300 mm, if a laser beam having a wavelength of 291 nm, shown by a dotted and dashed line in FIG. 15, exists, the incident radius R can be decreased to approximately 28 mm, but a laser beam having a wavelength of less than 325 nm does not actually exist at present.
Furthermore, the decrease in the incident radius R invites an increase of the focal length l. This becomes particularly serious when a wider range of scanning is needed, i.e., when an increased width of scanning is needed, since it results in a reduced straightness of the locus and in an increased aberration of the scanning beam. As a result, the beam scanner can not be used for a high precision scanning, as in, for example, a laser printer.